Ultrathin nickel oxide films grown on Ag(0 0 1): A study by XPS, LEIS and LEED intensity analysis

Article abstract of DOI:10.1016/S0039-6028(03)00544-2

The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was prepared by evaporating Ni in presence of O₂ at a pressure in the 10^-6 mbar range. The growth of the oxide film was followed by XPS, LEIS and LEED. In the early stages of deposition, the film shows a (2 × 1) superstructure in LEED. After deposition of 2 ML of NiO, a sharp (1 × 1) LEED pattern is observed. The intensity versus electron energy curves of the LEED spots were measured for this NiO(1 × 1) film and analysed by means of the tensor LEED method. A good level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was obtained. We found that oxygen atoms at the oxide-substrate interface are on-top silver atoms. The interlayer distance in the oxide does not differ significantly from that in bulk NiO(001), within the accuracy of the analysis. An outward displacement (0.05 ± 0.05 A?) of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The interlayer distance at the silver-nickel oxide interface is 2.43 ± 0.05 A?.

Full text of DOI:10.1016/S0039-6028(03)00544-2

Surface Science 531 (2003) 368–374
www.elsevier.com/locate/susc
Ultrathin nickel oxide films grown on Ag(0 0 1): a study
by XPS,LEIS and LEED intensity analysis
a
a
a
b,
c
M. Caffio ,B. Cortigiani ,G. Rovida ,A. Atrei *,C. Giovanardi ,
c
A. di Bona ,S. Valeri
c,d
a
Dipartimento di Chimica, Universitaꢀ di Firenze, Polo Scientifico di Sesto F.no. 50019, Sesto F.no (Fi), Italy
b
Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, Universitaꢀ di Siena, Via A. Moro, 53100 Siena, Italy
c
INFM-National Research Center on Nanostructure and Biosystems at Surfaces (S3) 41100, Modena, Italy
d
Dipartimento di Fisica, Universitaꢀ di Modena e Reggio Emilia 41100, Modena, Italy
Received 17 March 2003; accepted for publication 3 April 2003
Abstract
The structure of a nickel oxide film 2 ML thick has been investigated by LEED intensity analysis. The NiO film was
prepared by evaporating Ni in presence of O2 at a pressure in the 10ꢀ6 mbar range. The growth of the oxide film was
followed by XPS,LEIS and LEED. In the early stages of deposition,the film shows a (2 ꢁ 1) superstructure in LEED.
After deposition of 2 ML of NiO,a sharp (1 ꢁ 1) LEED pattern is observed. The intensity versus electron energy curves
of the LEED spots were measured for this NiO(1 ꢁ 1) film and analysed by means of the tensor LEED method. A good
level of agreement of the experimental LEED intensities with those calculated for a pseudomorphic NiO(0 0 1) film was
obtained. We found that oxygen atoms at the oxide–substrate interface are on-top silver atoms. The interlayer distance
in the oxide does not differ significantly from that in bulk NiO(0 0 1),within the accuracy of the analysis. An outward
ꢁ
displacement (0.05 ꢂ 0.05 A) of oxygen atoms with respect to nickel atoms was found at the oxide film surface. The
ꢁ
interlayer distance at the silver–nickel oxide interface is 2.43 ꢂ 0.05 A.
Ó 2003 Elsevier Science B.V. All rights reserved.
Keywords: Nickel oxides; Silver; Epitaxy; Growth; Surface structure,morphology,roughness,and topography; Low energy electron
diffraction (LEED)
1. Introduction
ence techniques [1–4]. Despite Ag(0 0 1) appears to
be an ideal substrate for the epitaxial growth of
nickel oxide (due to the small lattice mismatch and
the scarce reactivity of silver towards O2),those
studies show that the NiO/Ag(0 0 1) is a rather
complex system. Submonolayers of nickel oxide
deposited by evaporating nickel in oxygen atmo-
sphere at room temperature form a (2 ꢁ 1) struc-
ture [1–3]. A study by means of Spot Profile
Analysis of Low-Energy Electron Diffraction
(SPA-LEED) shows that upon increasing the NiO
The growth mechanism and structure of nickel
oxide ultrathin films prepared by evaporation of
nickel in oxygen atmosphere on Ag(0 0 1) have
been investigated by means of several surface sci-
* Corresponding author. Tel.: +39-0577-234371; fax: +39-
0577-234177.
E-mail address: atrei@unisi.it (A. Atrei).
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0039-6028(03)00544-2

M. Caffio et al. / Surface Science 531 (2003) 368–374
369
coverage the (2 ꢁ 1) phase is transformed into a
(1 ꢁ 1) phase [4]. The (1 ꢁ 1) islands have a double
layer height so that a film,which covers completely
the substrate surface,is obtained for a coverage
of two monolayers (ML). Above this coverage a
layer-by-layer growth was observed [4]. According
to these authors,the NiO film is pseudomorphic
up to a thickness of about 5 ML. For higher
coverages,the strain is released by the formation
of misfit dislocations [4]. Very recently ab initio
calculations performed for 1 ML of NiO(0 0 1) on
Ag(0 0 1) have been reported in the literature [5].
In the present work we studied by means of
LEED intensity analysis the structure of a nickel
oxide film 2 ML thick. We chose this thickness
since it corresponds to the situation when the
substrate surface is completely covered by a film of
NiO. Moreover,the thickness of the film is low
enough that the LEED intensities are sensitive to
the registry of the oxide layer with respect to the
substrate. This information can be compared with
the results of the calculations [5] as well as with the
experimental findings obtained by other tech-
niques [6,7].
cycles of argon ion sputtering (600 eV) and an-
nealing (700 K for 30 min) until no contamination
was detectable by means of XPS and LEIS and a
sharp (1 ꢁ 1) LEED was visible.
The LEED I–V curves were measured at normal
incidence of the electron beam in the energy range
50–350 eV. Normal incidence was considered to be
achieved when reliability factors (Pendry R-factor
[9]) between I–V curves of symmetrically equiva-
lent beams of 0.1 or below were obtained. The I–V
curves of symmetric beams were averaged to
compensate for minor differences due to small de-
viations from normal incidence. The I–V curves
were background subtracted and normalized to
constant incident electron current. The LEED in-
tensities were collected at room temperature.
Nickel (99.999 % purity) was evaporated by means
of an electron beam evaporator in the presence of
oxygen at a pressure of 3 ꢁ 10ꢀ6 mbar. During
evaporation the sample was held at 370 K. The
amount of nickel deposited was determined from
the area of the Ni 2p and Ag 3d XPS peaks. The
attenuation lengths of Ni 2p and Ag 3d were cal-
culated using the Tanuma Powell Penn method
[10]. The Ni 2p and Ag 3d photoionization cross
sections were taken from Ref. [11]. The evapora-
tion rate was estimated to be 0.2 ML of nickel
oxide per minute.
2. Experimental and calculation procedure
2.1. Experimental
2.2. TLEED calculations
The experiments were carried out in a ultra-high
vacuum chamber equipped with ion gun,X-ray
source and hemispherical analyzer for the sample
preparation and characterization. Non-mono-
chromatized AlKa radiation was used for X-ray
photoelectron spectroscopy (XPS). The XPS
spectra were measured at normal emission with a
fixed pass energy of 44 eV. The inelastic back-
ground in the spectra was subtracted by means of
the Shirley method [8]. For low-energy ion scat-
tering (LEIS) we used a He ion beam with an en-
ergy of 1 keV impinging on the surface at an angle
of 45°. The scattering angle was 135°. The LEED
measurements were performed by means of a
three-grid rear view LEED optics. The intensities
versus accelerating voltage (I–V ) curves of the
diffracted beams were collected by means of a vi-
deo LEED system. The surface was prepared by
The tensor LEED (TLEED) method consists of
a first step in which a full dynamical calculation is
performed for a chosen reference structure to
produce the elements of the tensor. With this
tensor,the I–V curves for various values of the
structural parameters in the neighborhood of the
reference structure can be fastly calculated,using a
perturbation method [12].
The TLEED calculations were performed by
means of the Barbieri/Van Hove Symmetrized
Automated Tensor LEED package (SATLEED)
[13]. The Barbieri–Van Hove program package
was used for the calculation of the phase shifts
[13]. Ten phase shifts ðlmax ¼ 9Þ were used in the
LEED calculations. The phase shifts for silver and
NiO were calculated from the muffin–tin potentials
of Ag and NiO. An additional set of phase shifts of

370
M. Caffio et al. / Surface Science 531 (2003) 368–374
Ni and O were calculated from the muffin–tin
potential of a 2 ML film of NiO(0 0 1) on Ag(0 0 1)
surface (to avoid problems with different values of
the inner potential of Ag and NiO). Test LEED
calculations performed with the two sets of phase
shifts lead to results which do not differ signifi-
cantly. The LEED calculations were performed
between 40 and 350 eV. The imaginary part of
inner potential was set to 5 eV,whereas the real
part was set to 10 eV and optimized in the search
procedure. The Debye temperatures were initially
set to 215,300 and 500 K for Ag,Ni and O re-
spectively and refined in the optimization process.
In the automated search of the structural param-
eters giving the best agreement with the experi-
mental I–V curves four non-equivalent beams were
used for a total energy range of about 630 eV. The
Pendry reliability factor ðRpÞ,was used in the
analysis to judge the agreement between experi-
mental and calculated I–V curves [9].
Fig. 1. Intensity of the LEIS signal of silver (normalized to that
of the clean surface) as a function of the amount of nickel oxide
as determined on the basis of the XPS data. The NiO film was
prepared by evaporating Ni in the presence of O2 at a pressure
of 3 ꢁ 10ꢀ6 mbar with the sample held at 370 K. The coverages
at which the strongest intensity of the (2 ꢁ 1) structure and
above which only the (1 ꢁ 1) pattern due to NiO can been seen
are marked by arrows. The solid line corresponds to the linear
decrease of the Ag LEIS signal with NiO coverage expected for
the growth a two-dimensional oxide film which covers com-
pletely the substrate surface after deposition of 1 ML. One
monolayer corresponds to one layer of NiO(0 0 1) (with the
3. Results and discussion
The growth of nickel oxide was followed by
means of XPS and LEIS. The Ag LEIS signal as a
function of the amount of deposited NiO (esti-
mated by XPS) is shown in Fig. 1. The Ag LEIS
signal is proportional to the fraction of substrate
not covered by the NiO oxide layer. In the early
stages of growth,the linear decrease of the Ag
LEIS signal with NiO coverage indicates the for-
mation of two-dimensional islands. Above 0.7 ML
the deviation from linearity suggests the growth of
a different phase. The substrate surface is com-
pletely covered by the NiO film for coverages
above 2 ML. These results are in agreement with
previous findings indicating that the Ag(0 0 1)
surface is entirely covered at a coverage of 2 ML
which corresponds to the onset of a layer-by-layer
growth mechanism [4]. The Ni 2p XPS spectra
measured for the (2 ꢁ 1) NiO film and (1 ꢁ 1) films
2 and 20 ML thick are shown in Fig. 2. The most
relevant changes of the Ni 2p peak position and
shape are observed in the earlier stages of growth.
Above 2 ML the Ni 2p line shape show only minor
changes upon increasing the film thickness and is
very similar to that measured for the NiO(0 0 1)
same in-plane parameter as the substrate) containing 0.12 Ni
2
ꢁ
atoms/ A and having a thickness of 2.1 A.
ꢁ
surface [14]. A detailed analysis of the Ni 2p line
shape as a function of the film thickness is reported
in Ref. [15].
For submonolayer coverages of NiO a (2 ꢁ 1)
LEED pattern is observed,in agreement with
previous studies [1,4]. The formation of the (2 ꢁ 1)
phase depends critically upon the ratio between O2
molecules flux and Ni atoms flux) [16]. Under our
experimental conditions the (2 ꢁ 1) extraspots
reach a maximum intensity at a NiO coverage of
ꢃ0.7–0.8 ML. Upon increasing the film thickness,
the reflexes of the (2 ꢁ 1) phase disappear and a
sharp (1 ꢁ 1) LEED pattern is observed. The de-
viation from linearity of the Ag LEIS signal versus
NiO coverage probably corresponds to the onset
of the growth of NiO(0 0 1)(1 ꢁ 1) islands.
The LEED I–V curves of the (1,0), (1,1) and
(2,0) beams measured for the NiO(2 ꢁ 1),NiO

M. Caffio et al. / Surface Science 531 (2003) 368–374
371
(2, 0)
(1x1)
(2x1)
clean
(1, 1)
(1,0)
Fig. 2. Ni 2p XPS spectra measured for a (2 ꢁ 1) NiO film
(bottom) and (1 ꢁ 1) NiO films 2 ML (middle) and 20 ML thick
(top). The intensities of each spectrum are normalized to the
maximum value and offset.
50 100 150 200 250 300
Energy (eV)
Fig. 3. LEED I–V curves of the (1,0), (1,1) and (2,0) beams
measured for the clean Ag(0 0 1) surface,the (2 ꢁ 1) and (1 ꢁ 1)
NiO films. The NiO coverage corresponding to the (2 ꢁ 1) phase
was 0.7 ML. The nominal coverage for the (1 ꢁ 1) NiO film was
estimated to be 2.4 ML.
(1 ꢁ 1) and clean Ag(0 0 1) surfaces are compared
in Fig. 3. The I–V curve of the integral order spots
for the (2 ꢁ 1) phase are very similar to those
measured for the clean Ag(0 0 1) surface. This is in
agreement with the LEIS data showing that when
the (2 ꢁ 1) LEED pattern is observed a significant
fraction of the substrate is not covered by the
oxide layer. On the other hand the I–V curves are
substantially modified by the presence of the 2 ML
thick NiO film. These results indicate that for a
reliable LEED analysis of the (2 ꢁ 1) phase only
the fractional order beams can be used. When only
the (2 ꢁ 1) oxide layer is present a contribution
to the intensity of the integral order spots comes
from the substrate. At higher coverages NiO(1 ꢁ 1)
islands are present and contribute to the integral
order beam intensity in a way which is difficult to
model.
ꢁ
(2.084 A). Three possible registries of the NiO ox-
ide film with respect to the substrate surface were
considered. Ni atoms on-top Ag atoms and O atoms
in hollow sites (model A). O atoms on-top Ag at-
oms and Ni atoms in hollow sites (model B). Both
O and Ni atoms occupy bridge sites (model C). In
the latter case the presence of two domains was
taken into account. The interlayer spacing at the
ꢁ
NiO/Ag interface was set to 2.70,2.40 and 2.40 A
for model A,B and C,respectively. These values
were calculated for O–Ag and Ni–Ag bond lengths
ꢁ
ꢁ
of 2.4 A and 2.7 A,respectively [51, 7]. Additional
reference structure calculations were performed in
a range of values around the NiO/Ag interlayer
spacings reported above. For each registry,the
calculations were performed for one layer,two and
three layers of NiO. In the search procedure of the
best fit with the experimental I–V curves,the
atomic coordinates perpendicular to the surface in
the NiO layers and in the topmost layer of the
substrate were allowed to vary. The minimum
values of the Rp obtained for the models described
The I–V curves measured after deposition of 2
ML of NiO were used in the tensor LEED analysis.
As basic structural model we considered
a
pseudomorphic NiO(0 0 1) film. In this structure
the in-plane lattice parameter is contracted by
ꢁ
2.28% with respect to the value (4.168 A) of
NiO(0 0 1). The interlayer distance between the Ni–
O planes was set equal to that of bulk NiO(0 0 1)

372
M. Caffio et al. / Surface Science 531 (2003) 368–374
Table 1
best fit is obtained for a film 2 ML thick. Starting
from the optimized coordinates,a new reference
structure calculation was performed. In addition to
the structural parameters,also the imaginary part
of the inner potential and the Debye temperatures
of the atomic species were optimized in the analy-
sis. After refining a minimum Rp of 0.18 was ob-
tained. On the other hand,the optimization of the
non-structural parameters for model A did not
decrease significantly the R-factor (Rp 0.27). The
structural and non-structural parameters for model
B are reported in Table 2. The calculated I–V
curves for the best fit model are compared with the
experimental curves in Fig. 5.
Minimum values of the Pendry R-factor ðRpÞ obtained from
calculations performed for pseudomorphic NiO(0 0 1) films of
various thickness
Registry
1 ML
2 ML
3 ML
A
B
C
0.68
0.60
0.66
0.28
0.22
0.40
0.36
0.33
0.33
For each thickness,the calculations were performed for three
registries of the NiO film with respect to the Ag(0 0 1) substrate.
A: Ni atoms directly above Ag atoms and O atoms in the
hollow sites. B: O atoms directly above Ag atoms and Ni atoms
in the in the hollow sites. C: Ni and O atoms in bridge positions.
above are reported in Table 1. The best fit is ob-
tained for a NiO film having a thickness of two
layers with O atoms on top Ag atoms at the in-
terface (model B,Fig. 4). The difference of the R
factors between model A and B is small but sig-
nificant. As expected,the differences amongst the
registries decrease upon increasing the thickness of
film. The results of the analysis are consistent with
the thickness of the film estimated by XPS since the
The interlayer spacing in the NiO film is very
close to the bulk value. The buckling in the out-
ermost NiO layer,consisting in an outward dis-
placement of oxygen atoms with respect to the
plane passing through the Ni atoms is comparable
to the uncertainty of the analysis. Since the in-
plane lattice parameter of the NiO film is com-
pressed,an expansion of the interlayer spacing
would be expected. Indeed,an expansion of the
interlayer distances (1.9 ꢂ 0.4%) for a pseudor-
morphic NiO film 3 ML thick grown of Ag(0 0 1)
was found by means of primary-beam diffraction
modulated electron emission (PDMEE) [7]. This
technique,which is similar to X-ray photoelectron
diffraction [18],was applied successfully to the
study of the MgO/Ag(0 0 1) interface [19]. The
expansion of the interlayer distance determined by
PDMEE is compatible with the LEED results,
[001]
[100]
d12
d23
Table 2
Structural and non-structural parameters corresponding to the
best fit structure
d34
d45
ꢁ
d12
d23
d34
d45
0.05 ꢂ 0.05 A
ꢁ
2.05 ꢂ 0.05 A
dAg
ꢁ
0.03 ꢂ 0.05 A
ꢁ
2.43 ꢂ 0.05 A
ꢁ
dAg
2.04 ꢂ 0.08 A
ꢁ
2.036 A
dAgð0 0 1Þ
Td,O
Td,Ni
Td,Ag
Vi
600 K
450 K
210
Ni
O
Ag
5 eV
Fig. 4. Cross section (along the [1 0 0] direction) of model B for
2 ML of NiO on Ag(0 0 1). The outward displacements of O
atoms is enlarged for clarity.
See Fig. 3 for the definition of the interlayer spacings. The error
bars of the structural parameters were determined from the
variance of Rp [9].

M. Caffio et al. / Surface Science 531 (2003) 368–374
373
is completely covered by this oxide film,islands of
NiO(0 0 1)(1 ꢁ 1) start to nucleate. The Ag(0 0 1)
surface is completely covered for coverages above
2 ML of NiO(0 0 1). The structure of the nickel
oxide film 2 ML thick has been determined by
means of TLEED analysis. The results of the
analysis confirm that NiO grows in a pseudo-
morphic way on Ag(0 0 1). The interlayer distances
in the NiO film are very close to value in bulk
NiO(0 0 1). We found that O atoms are directly
above Ag atoms at the interface. The present re-
sults are in very good agreement with the theo-
retical findings for 1 ML of NiO on Ag(0 0 1) and
with the PDMEE results obtained for a NiO film 3
ML thick. This would suggest that the relaxations
of the interlayer spacings are not strongly influ-
enced by the thickness of the NiO film.
Acknowledgements
Financial support by Istituto Nazionale per la
Fisica della Materia (Advanced Research Project
ISADORA) and by Ministero dellÕIstruzione,
Fig. 5. Comparison of the experimental LEED I–V curves with
those calculated for the best fit model. For each beam the
corresponding value of Rp is reported.
ꢀ
dellÕUniversita e della Ricerca (Progetti FIRB) is
gratefully acknowledged.
considering the error bars of the structural pa-
rameters and the buckling in the outmost NiO
layer. The PDMEE results for a 3 ML NiO film
grown on Ag(0 0 1) suggest that oxygen atoms are
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